Taking the adventures of clean and sustainable, hydrogen has emerged as potential energy carriers for addressing the energy consumption problem. How to obtain the fuel has already been a vital issue in the decade. Among several methods, electrolysis provides an efficient way to convert electricity into chemical bonds. In industry applications, platinum is used as working electrodes for hydrogen production. However, its high cost and energy inefficiency limit the widespread application and encourages people to seek for earth-abundant elements as substitutes as well as highly efficient catalysts for hydrogen evolution reaction (HER).
In nature, [FeFe]hydrogenase enzymes have catalytic efficiency of 6000-9000 molecules of molecular hydrogen per second (Frey, M. Chembiochem 2002, 3, 153). The active site of [FeFe]hydrogenase is composed of a diiron unit with ligation of carbonyl, cyanide, a dithiolate linker and a [4Fe4S] cluster. Intensive synthetic study on modeling complexes has been performed to mimic the active site of [FeFe]hydrogenase in order to replicate the catalytic ability for hydrogen production. FIG. 1 is a schematic diagram that summarizes several types of biomimetic models to the active site of [FeFe]hydrogenase via modification of redox subunits, ligands and sub-site structures. For instance, the attached [4Fe4S] cluster is replaced by an artificial redox active unit to enhance the electron communication between the diiron core and the protein backbone (Type 1). The dithiolate bridge is substituted by different abiological analogs (Type 2). The first coordination sphere about the Fe center is replicated by coordination of σ-donating ligands. The bridgehead atom is changed from nitrogen to carbon, oxygen or other chalcogen atoms (Type 3). A vacant site, an iron-hydride and a bridging/semi-bridging CO group are key structural features in intermediates (Types 4-6). These synthetic approaches are employed to improve the performance of electrocatalysis for hydrogen formation. The catalytic efficiency unfortunately remains low albeit these studies have been investigated for over a decade. In 2012, Rauchfuss et al. reported a diiron thiolate complex that reveals a high turnover frequency (TOF) of 58,000 s−1 (Carroll, M. E.; Barton, B. E.; Rauchfuss, T. B.; Carroll, P. J. J. Am. Chem. Soc. 2012, 134, 18843).
In addition, several homogeneous electrocatalysts, such as nickel, cobalt, and molybdenum complexes, have been developed for the purpose of the production of hydrogen. The highest TOF number of 106,000 of H2 per second is reported by DuBois et al. in 2011. A synthetic nickel complex is employed in the presence of medium strength acids ([(DMF)H]OTf, pKa=6.1 in acetonitrile) as proton sources (Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. Science 2011, 333, 863).